Extended Bogy Testing

Introduction

Extended bogy testing builds on test to bogy (TTB), discussed in a prior article. TTB focused on calculating the number (N) of parts tested to one life bogy, with 0 failures allowed, to a specified reliability (R) and confidence (C) levels.

Using TTB to verify conformance to high reliability and confidence targets requires very large sample sizes, increasing testing cost. The capacity to test large samples may require large facility capital expenditures. Also, the zero failures allowed paradigm removes the opportunity to learn about product failure modes and the opportunity to improve the product through design or manufacturing process changes.

This article focuses on extended bogy test plans as an alternative to TTB.

Determine Success Testing Sample Size

“How many samples do we need?” is a very common question. It is one you will receive when planning nearly any kind of reliability testing. It is a great question.

Having too few samples means the results are likely not useful to make a decision. Too many samples improve the results, yet does add unnecessary costs. Getting the right sample size is an exercise starting in statistics and ending with a balance of constraints.

There are six elements to consider when estimating sample size. We will use the success testing formula, a life test with no planned failures, to outline the necessary considerations. [Read more…]

The Eyring Model for Accelerated Testing

Sometimes the reaction rate of a process relies on two stresses. For chemical reactions temperature seems to influence the rate of the reaction. Yet, other stresses such as humidity or voltage may also play a significant role.

H. Eyring suggested a model that assumes the contribution of each stress on the reaction rate is independent thus one could multiple the respective stress contributions to the rate of reaction.

The Erying model provides a means to account for the contributions of temperature and another stress when modeling the time to failure of select failure mechanisms. [Read more…]

Years ago I learned from a former Apple reliability group manager how to organize reliability and environmental related testing where samples cascade through a sequence of stress conditions and evaluations. He called it waterfall testing. [Read more…]

4:2:1 Allocation of Test Units

One question that you should consider when planning a multiple stress accelerate life test (ALT) is the allocation of test units to the various stresses.

We want to create a model detailing the relationship between stress and time to failure. We also want to project the time to failure estimates to use conditions. Ideally, we test at nominal conditions only and gather time to failure information. We do not have the luxury of time thus explore using ALT.

One method of allocation is to place an equal number of samples with each stress level. Is that the best approach? [Read more…]

Black’s Equation for Electromigration Accelerated Life Testing

Black’s equation for estimating the time to failure due to electro migration is a classic. James Black explored and wrote about electromigration in aluminum metallization within semiconductors since 1969.

He and others have explored other materials used as conductor prone to electromigration. Thus, there are a number of models and constants available to match your particular system.

Let’s take a look at the general equation for a microcircuit conductor after a brief description of the failure mechanisms called electromigration. [Read more…]

Metal Fatigue Failure Mechanism Accelerated Life Testing

Metal is a wonderful, strong, material. Yet under certain types of stresses metal can fail One in particular is fatigue due to cyclic motion.

Metals in a solid state have an atomic level lattice structure. This provides the strength and flexibility. It is the flexibility part that causes trouble. We don’t get the benefit of flexibility for free. As the metal bends it ‘adjusts’ the lattice to accommodate the motion. In doing so, it changes the metal properties becoming a bit more brittle, for example.

In most cases a very small motion causes imperceptible changes and loss of functionality. In some cases, like bending a wire coat hanger with the intent to break it, just a few cycles of dramatic bending is enough to break the wire.

In metal applications that experience cyclic motion and the risk of metal fatigue failure may occur during the expected duration of product use, we may need to characterize the time to failure behavior. An accelerated life test for a metal fatigue failure mechanism is not difficult, yet does take some planning to get meaningful results. [Read more…]

Peck’s Relationship for Temperature & Humidity Testing

High temperature & humidity is a common test condition. For specific failure mechanisms, there are models available (or you can create a model) to determine the translation from test to use conditions.

These acceleration models generally only apply to one specific failure mechanisms and do not apply to a system level estimate of life. If the failure mechanism is the dominant failure mechanism for the product, then an ALT exploring just that mechanisms would provide a life estimate.

Peck’s relationship is an acceleration model for the effect of humidity on the metallization elements of integrated circuits within plastic enclosures (typically an epoxy over molding). [Read more…]

Time Compression ALT – The Easy One

The easiest ALT is one that you operate an item more often then operated by the customer. Removing spans of time the item is not being bent, moved, heated, etc allows you to use time compression.

For example, a home kitchen toaster may be used for a few cycles during breakfast time in your home. In the lab, we can avoid having to wait the day of idle time and just make toast more often than just at breakfast to accelerate the operation of a toaster.

Time compression ALT is also easy to understand and describe the acceleration factor to cover the ALT results to field use conditions. Let’s explore a simple example, work out the acceleration factor and how to interpret a set of ALT results. [Read more…]

Getting the Right Information from Your Reliability Testing

You cannot test in reliability any more than you can test in quality. Often reliability testing is done though, and knowing the range of testing approaches and their associated results will help you get the most information from each test conducted.

Let’s explore the types of testing that generates information useful as you develop a reliable product. There are 4 different types of reliability testing:

Discovery

Life

Environmental

Regulatory

Within each type there are many variations to the testing details and the specific results generated. Understanding the questions each type of testing has the capability to resolve is a good first step to implementing the right set of tests for your project. [Read more…]

HALT in 4 Not Always Easy Steps

Highly Accelerated Life Testing, HALT, is a method to discover the weaknesses in a design. Using a step stress approach of single and combined stresses, you can quickly expose the salient weaknesses in your design and/or assembly process.

The value of HALT is it’s quick and often finds problems not previously known. You will destroy one or more prototypes, yet the value of knowing specifically what needs improvement more then justifies the sacrifice of a few photos.

Conducting HALT may be part of your reliability plan. Keeping a few steps in mind will help make sure your HALT does provide value back to your development efforts. [Read more…]

Two Birds with One Stone – A Deadline and A Question

Just back from a trip to Patagonia and catching up with emails and writing this morning. Posting an article for this list is due today along with a touch of travel weariness, decided to share a part of a question received concerning data analysis.

My thought is to post an actual question one of our peers is facing, and meet the deadline for this post. [Read more…]